1. Field of the Invention
The invention relates to electronic communication and, more particularly, to the synchronization of a pseudo-random noise sequence in a direct-sequence spread-spectrum communications system.
2. Description of the Related Art
Cordless telephones are generally known in the art and are popular with residential and individual consumers. As cordless telephone technology advances, cordless telephones may also prove advantageous to other consumers, such as businesses and commercial groups. When cordless telephones are designed for the lower-end residential and individual consumer market, price and quality are primary considerations of those consumers. Digital telephones tend to provide greater sound quality and capabilities than analog telephones. It is desirable, therefore, that a digital cordless telephone of good quality and adequate capabilities be available to that lower-end market. The cordless telephone market is particularly price-conscious. Low-end consumers, such as residential and individual users, particularly look for economy. Although various designs of digital cordless telephones may be available, those designs have not adequately met the consumer's need for quality as well as economy. A digital cordless telephone that meets those expectations of consumers would thus provide significant improvement and advance in the technology.
Beyond those two expectations of quality and economy of cordless telephone consumers, residential and individual cordless telephone users must typically operate within a limited bandwidth. This restriction presents problems that must be addressed by digital cordless telephone designers. For example multiple users may need to simultaneously communicate within the narrow bandwidth. In order to avoid interference among users and inaccurate communications in those cases, designs of digital cordless telephones must account for this multiple user scenario. This is complicated by the fact that those designs must also meet market requirements such as quality and low price, as previously described.
Certain newer cordless telephones are employing spread spectrum technology. Direct sequence spread spectrum technology involves spreading the narrowband communications signal over a wide frequency band, thus reducing the amount of power in each portion of the frequency band. The principle advantage of spread spectrum transceivers in the United States is the ability to transmit at greater power levels in the 902-928 MHz ISM band under FCC regulations, thereby attaining greater range of handset mobility with respect to the base as compared to lower-power narrowband transmissions. Other advantages of this spreading include an improved rejection of interference signals, and a greater resistance to multi-path fading--which can cause a handset to lose contact with a base unit in certain volumes of space. Current spread-spectrum cordless telephone solutions utilize inherently expensive architectures or compromise performance in order to reduce cost.
One particular aspect of the spread spectrum cordless telephones that bears further improvement is the synchronization of a pseudo-random noise (PN) sequence in the receivers of systems that employ time-division duplexing (TDD). The PN sequence, also known as a "spreading sequence" or "spreading code," is a sequence of values, called "chips", each with a duration substantially shorter than the duration of the information symbols in the transmitted signal. The transmitted signal is modulated with the PN sequence, thereby spreading the frequency spectrum of the transmitted signal.
In some direct sequence spread spectrum communications transceivers the PN sequence is a repeated finite sequence of binary values (+1's or -1's). The length of the sequence varies between implementations. The repeated sequence preferably has the three randomness properties of balance, run, and correlation. These three properties give the repeated sequence a resemblance to a random sequence. The balance property of the repeated sequence is that it should have an equal number of high and low values. Ideally, the number of +1's in the repeated sequence differs from the number of -1's by at most one. The run property of the repeated sequence concerns the grouping of consecutive +1's or consecutive -1's in the sequence. Each grouping of consecutive values is called a "run." Among the runs of the repeated sequence, preferably about one-half have length one, about one-quarter have length two, one-eighth have length three, etc. The correlation property of the repeated sequence dictates that if the repeated sequence is compared term-by-term with a shifted version of itself, then about half of the comparisons are agreements, and half are disagreements. That is, the autocorrelation function of the repeated sequence is strongly peaked at zero shift.
In order for a receiving unit to de-spread the received spread spectrum signal, the receiving unit must have a receiver PN sequence that is synchronized with the PN sequence in the received signal. That is, each of the repeated sequences in the receiver PN sequence must start at the same time as the repeated sequences encoded in the received signal. Put another way, the phase of the receiver PN sequence must match that of the PN sequence in the received signal. With this synchronization, the receiver can demodulate the binary PN sequence from the received signal and regenerate the original narrowband signal. The process of synchronizing the receiver PN sequence to the PN sequence in the received signal is PN timing recovery.
The field of digital communication has evolved a variety of techniques for performing the PN timing recovery. Principal among these is the "maximal likelihood" or "sliding correlator" method, which measures correlations between the received signal and a locally generated receiver PN signal. The receiver PN signal is progressively phase-shifted until a peak correlation is detected. This method has several disadvantages that cause it to be prone to false detection of synchronization. This method is especially lacking in TDD communications systems, in which two transceivers communicate on a single frequency channel by alternating between transmitting and receiving data. In a TDD receiver, the maximal likelihood method is susceptible to false synchronization resulting from the rapid change in received signal power as the remote transceiver switches from receive to transmit modes. Thus, it would be desirable to have a robust hardware-implemented system for acquiring the PN timing in a direct-sequence spread-spectrum TDD communications system.
Improvements can also be made to the techniques for synchronizing the frame timing in TDD receivers with the frame sequence in the received signals. This synchronization is typically performed by constructing the transmitted frames with a SYNC field--a predetermined fixed pattern of data that occurs repeatedly in the same position in the frame. The SYNC field may be included in every transmitted frame, allowing a robust measure of synchronization, or it may be included less often, allowing greater data transmission rates. The receiver monitors the received data for the SYNC pattern, and upon detecting it, sets the frame timing accordingly. After the frame timing is set, the receiver may continue to monitor the data to verify that the SYNC pattern occurs in the expected positions.
Current receivers typically monitor the received data for the SYNC pattern using the same high-level systems that read the desired data from the received frames. It would be desirable to have a more self-contained system for synchronizing the receiver with the frame timing in the received signal. Such a system would independently acquire and maintain the frame synchronization without disrupting other systems and functions of the receiver.